Emission patterns of volatile organic compounds from Norway spruce logs following bark beetle (<i>Ips typographus</i> L.) infestation

Norway spruce (Picea abies L.) is economically one of the most important conifer species in Europe. Spruce forests are threatened by outbreaks of the bark beetle Ips typographus L., and this will worsen with a projected warmer and drier climate and increased outbreak dynamic following storms. Volatile terpenes and aromatics play pivotal roles in defence of trees, however little is known about the emission dynamics of these compounds from trees colonized with I. typographus, particularly in dependence on the beetle's developmental stages and colonization densities. Here, we analysed the emission profiles of volatile organic compounds (VOCs) from spruce logs colonized with low (LDT) and high (HDT) densities of I. typographus from the initial colonization until the emergence of the next beetle generation. A first VOCs emission peak appeared directly after colonization and lasted for 1 week. It mainly consisted of monoterpenoids and most likely reflected the trees' constitutive defence against herbivory. Under HDT, a second emission maximum occurred during the larval stage, whereas under LDT a second peak appeared later during maturation feeding of callow beetles. In contrast to the first peak, sesquiterpenoids, aromatics and oxygenated monoterpenoids dominated the second peak, possibly reflecting the trees' induced defence. Developing beetles seem to face a variety of defence compounds even if the tree has been overwhelmed and colonization by parental beetles was already successful. The specific release patterns under LDT and HDT might be due to different availability of precursor compounds, depletion of constitutively stored compounds and differences in microbial activities associated with the bark beetles. The present study highlights constitutive defence of spruce trees upon herbivore attack, and the effect of infestation density on the temporal dynamics of induced defence. Particularly the oxygenated monoterpenoid terpinen-4-ol may be used as cue to assess habitat quality and competition by newly arriving beetles.